As for a damper wherein relative to a case having an annular space, a cylindrical rotor is rotatably housed in the annular space, and a resistance to a rotation of the rotor is provided by the viscous fluid filled in the annular space, there are dampers shown in Patent Document 1 and Patent Document 2.
However, in the damper in the Patent Document 1, an O-ring which seals between an outer side of the rotor and the case, and an O-ring which seals between an inner side of the rotor and the case are positioned to overlap on an imaginary line segment perpendicular to a center axis of the rotation of the rotor to cause difficulty in minimizing an external diameter of the damper.
On the other hand, in the damper in the Patent Document 2, in order to hold the O-ring sealing between the outer side of the rotor and the case, there is required a cap. Also, a seal portion by the O-ring in the rotor does not have a structure allowing the external diameter of the damper to be minimized.